Add an option to use user/system hgrc files

By default, the plugin ignores user and system .hgrc files by setting
the HGPLAIN environment variable.

Occasionally, a user may want to use /home/jenkins/.hgrc to configure
something the plugin doesn't have an explicit option for.

This gives the user an option to use user and system .hgrc files, at
their own risk.  This is off by default and I added a caution message to
the help file.
Add option to disable changelog calculation

In some very very large repositories, the changelog calculation can take
quite a bit of time.  For users that do not care about seeing the
changelog in Jenkins, this allows disabling the calculation, thus
speeding up builds.

JENKINS-16654